The present invention relates to a microstrip line which is used in consumer microwave receivers for receiving signals of satellite broadcasting, satellite communications and the like, and to a microwave device using the microstrip line.
In consumer microwave receivers (for satellite broadcasting, satellite communications and so on) which use the 12 GHz band, microwave signals received from a receiving antenna are very weak. Such signals are so weak that they are sent directly to a low noise amplifier through a microwave transmission line in order to be minimally attenuated (by circuit loss, for example), and to be minimally imposed by noise. The microwave transmission line and the low noise amplifier are located in a block down-converter which is usually coupled to the receiving antenna. The microwave transmission line has usually been a microstrip line which is constructed by a printed circuit board (hereinafter PCB) using a low loss dielectric substrate.
FIG. 5 shows an example of a conventional microstrip line which is constructed by a PCB made of the low loss dielectric substrate. As shown in FIG. 5, the microstrip line is formed in PCB 2 in such a manner that conductive foil 1 for grounding and conductive foil 3 for strip conductor are disposed on the back surface and the front surface of PCB 2, respectively.
The thickness of PCB 2 for the 12 GHz band is 1 mm or less, so that PCB 2 may be deformed by mechanical and thermal distortions. PCB 2 is held in contact with the face of metal plate 4 for grounding. Plate 4 protects the microstrip line from property degradation caused by the distortions.
It is theoretically known that width 5 of strip conductor foil 3 is mainly determined by operating frequency, desired characteristic impedance, relative dielectric constant of the dielectric substrate, and thickness of conductor foil 3.
For the purpose of the use mentioned above, transmission loss in the microstrip line is preferred to be reduced to a minimum. It is known also that the transmission loss includes dielectric loss of the PCB, copper loss of the strip conductor, and radiation loss from the strip conductor. In particular, the dielectric loss of the PCB and the radiation loss from the strip conductor become remarkable in the microwave frequency band.
Though a teflon-fiber substrate with low dielectric loss has been widely used for conventional PCB 2, the teflon-fiber substrate is relatively so expensive that a microwave device using a teflon-fiber substrate has been also expensive.
On the other hand, glass-epoxy substrate for a low cost PCB in common use in a lower frequency than the microwave has not only high dielectric dissipation factor (tan xcex4) but also large relative dielectric constant of about 4 larger than that of 2 of the teflon-fiber. Therefore, when a microstrip line is made of a glass-epoxy substrate with the. same thickness and the same characteristic impedance as it is made of a teflon-fiber substrate, the width of the strip line made of the glass-epoxy substrate is smaller than the width of the microstrip line made of the teflon-fiber substrate. Consequently, the loss in the microstrip line disposed on the PCB of glass-epoxy is increased by copper loss of the strip conductor in addition to the dielectric loss of the PCB.
A microstrip line comprises a PCB, a strip conductor foil which is disposed on the front of the PCB for microwave transmission (hereinafter referred to as strip conductor), and a metal plate for grounding which is disposed in contact with the back of the PCB (hereinafter referred to as ground plate). The ground plate has a groove running along the strip conductor facing towards the strip conductor, and the groove acts as an air layer formed between the PCB and the ground plate. The microstrip line having a structure mentioned above is regarded as a microstrip line having a combined dielectric layer into which the PCB and the air layer are combined. The effective complex relative dielectric constant is determined according to a ratio of thickness of the PCB and thickness of the air layer, and can be set to a desired intermediate value between the two complex relative dielectric constants of the PCB and the air layer. Since it can be safely said that the complex relative dielectric constant of the air layer is practically the same as that of the vacuum, i.e. xe2x80x9c1+j0xe2x80x9d. In other words, both the real part and the imaginary part of the effective complex relative dielectric constant become smaller than those of the PCB itself. Thus the decrease in the real part of the effective complex relative dielectric constant allows the width of the strip conductor to be increased and allows copper loss to be decreased. Furthermore, the decrease in the imaginary part of the effective complex relative dielectric constant allows the effective dielectric loss of the combined dielectric layer to be decreased.
A microwave transmission line between an input terminal and an impedance matching circuit and the impedance matching circuit are constructed by a microstrip line which has a strip conductor, a dielectric substrate, an air layer, and a ground plate, so as to amplify input microwave signal and keep noise figure at a low level.
Furthermore, using a waveguide probe for a port of input terminal taking microwave signal out of a waveguide enables to make a small-sized, low-loss, and high-performance microwave device.